ALSPAC also has some weaknesses. Despite its size, it lacks power to study rarer exposures and outcomes. Although ALSPAC has collected repeat measures across frequent time points, the early (up to the age of 5 years) collection of data at study assessment clinics was limited to a 10% subsample. Nevertheless, the data collected have been very informative in regards to early growth and development. Collecting data from young adults is notoriously difficult and it is no surprise that ALSPACs response rates have decreased over time. To address this, ALSPAC is investing resources into collecting data from health and administrative records as well as participation initiatives aimed at increasing response. Incomplete recruitment and subsequent attrition have further reduced power and the availability of repeat measures across multiple time points and may have introduced bias in relation to estimation of some effects. Moreover, the demographic profile of the catchment area population and the effects of subsequent differential attrition have led to an over-representation of more affluent groups and an under-representation of non-White minority ethnic groups compared with the national population. This may
